Adhesives for frozen substrates

ABSTRACT

The use of free-radically polymerized, UV-crosslinkable polymers which consist to the extent of at least 50% by weight of C 2  to C 18 alkyl(meth)acrylates as adhesives for the bonding of carriers coated with the polymer on substrates, wherein from 0.1 to 30% by weight of the monomers of which said polymer is composed are monomers A without carboxylic acid or carboxylic anhydride groups and with a water solubility of more than 5 g of monomers per liter of water and said substrates are moist substrates, especially refrigerated substrates.

FIELD OF THE INVENTION

The invention relates to the use of free-radically polymerized,UV-crosslinkable addition polymers which consist to the extent of atleast 50% by weight of C₂ to C₁₈ alkyl(meth)acrylates as adhesives forthe bonding of carriers coated with the polymer on substrates, whereinfrom 0.1 to 30% by weight of the monomers of which said polymer iscomposed are monomers A without carboxylic acid or carboxylic anhydridegroups and with a water solubility of more than 5 g of monomers perliter of water and said substrates are moist substrates, especiallyrefrigerated substrates.

BACKGROUND OF THE INVENTION

UV-crosslinkable polymers and their use as adhesives—as hot-meltpressure-sensitive adhesives, for example—are known from DE-A-2 411 169,EP-A-246 848, DE-A-4 037 079 or DE-A-3 844 444, for example.

These adhesives have not been used to date for moist substrates,especially refrigerated substrates.

For producing labels for refrigerated product it is normal to use blockpolymers of the styrene-isoprene-styrene or styrene-butadiene-styrenetype. A general disadvantage of these block copolymers lies in theirsoftness, which leads to difficulties in the course of processing anduse.

There is a desire for alternative polymers as adhesives for moist,refrigerated substrates.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide alternative polymersfor such a use.

We have found that this object is achieved by the use defined at theoutset.

For crosslinking with UV light, the addition polymer may contain aphotoinitiator. The photoinitiator may be attached to the polymer butmay also be unattached and merely mixed with the polymer.

Customary photoinitiators that can be added to the polymer are, forexample, acetophenone, benzoin ethers, benzil dialkyl ketals, orderivatives thereof.

The amount of photoinitiator mixed in is preferably from 0.05 to 10parts by weight, with particular preference from 0.1 to 2 parts byweight, per 100 parts by weight of polymer.

Through irradiation with high-energy light, especially UV light, thephotoinitiator or photoinitiator group brings about a crosslinking ofthe polymer, preferably by means of a chemical grafting reaction of thephotoinitiator group with a spatially adjacent polymer chain.Crosslinking can be effected in particular by inserting a carbonyl groupof the photoinitiator into an adjacent C—H bond to form a —C—C—O—Hgrouping.

The wavelength range within which the photoinitiator group can beactivated, i.e., that in which the principal absorption band of thephotoinitiator group lies, is preferably from 200 to 450 nm, withparticular preference from 250 to 350 nm, with very particularpreference from 250 to 280 nm.

Preferably, the photoinitiator is attached to the polymer.

The polymer is obtainable by free-radical addition polymerization fromethylenically unsaturated free-radically polymerizable compounds.

In the preferred case where the photoinitiator is attached to thepolymer an ethylenically unsaturated compound having a photoinitiatorgroup is preferably incorporated by copolymerization.

DETAILED DESCRIPTION OF THE INVENTION

The UV-crosslinkable polymer consists preferably to the extent of from50 to 99.85% by weight, preferably from 60 to 99.4% by weight, with veryparticular preference from 80 to 98.9% by weight, of C₂ to C₁₈alkyl(meth)acrylates. Preference is given to C₂-C₁₀alkyl(meth)acrylates, e.g., n-butyl acrylate, ethyl acrylate, and2-ethylhexyl acrylate. Mixtures of the alkyl(meth)acrylates are used inparticular.

In the case of the copolymerized photoinitiator the addition polymerfurther consists to the extent of from 0.05 to 10% by weight, preferablyfrom 0.1 to 2% by weight and with particular preference, from 0.1 to 1%by weight, of ethylenically unsaturated compounds having aphotoinitiator group.

The ethylenically unsaturated compound having a photoinitiator group ispreferably an acetophenone derivative or, with particular preference, abenzophenone derivative.

Suitable compounds are acetophenone or benzophenone derivativescontaining at least one—preferably one—ethylenically unsaturated group.The ethylenically unsaturated group is preferably an acrylic ormethacrylic group.

The ethylenically unsaturated group can be attached directly to thephenyl ring of the acetophenone or benzophenone derivative. In general,there is a spacer group situated between phenyl ring and ethylenicallyunsaturated group.

The spacer group can contain, for example, up to 100 carbon atoms.

Suitable acetophenone or benzophenone derivatives are described, forexample, in EP-A-346 734, EP-A-377199 (1^(st) claim), DE-A-4 037 079(1^(st) claim) and DE-A-3 844 444 (1^(st) claim) and are incorporated bythis reference into the present specification. Preferred acetophenoneand benzophenone derivatives are those of the formula

where R¹ is an organic radical having up to 30 carbon atoms, R² is ahydrogen atom or a methyl group, and R³ is an unsubstituted orsubstituted phenyl group or a C₁-C₄ alkyl group.

With particular preference, R¹ is an alkylene group, especially a C₂-C₈alkylene group.

With particular preference, R³ is a methyl group or a phenyl group.

The addition polymer may further include ethylenically unsaturatedcompounds as structural components. Mention may be made, for example, ofvinyl esters of carboxylic acids containing up to 20 carbon atoms,vinylaromatics having up to 20 carbon atoms, ethylenically unsaturatednitriles, vinyl halides, vinyl ethers of alcohols containing 1 to 10carbon atoms, aliphatic hydrocarbons having 2 to 8 carbon atoms and 1 or2 double bonds, or mixtures of these monomers.

Examples of vinyl esters of carboxylic acids having 1 to 20 carbon atomsare vinyl laurate, vinyl stearate, vinyl propionate, Versatic acid vinylesters, and vinyl acetate.

Suitable vinylaromatic compounds are vinyltoluene, α- andp-methylstyrene, α-butylstyrene, 4-n-butylstyrene, 4-n-decylstyrene and,preferably, styrene. Examples of nitriles are acrylonitrile andmethacrylonitrile.

The vinyl halides are chloro-, fluoro- or bromo-substitutedethylenically unsaturated compounds, preferably vinyl chloride andvinylidene chloride.

Examples of vinyl ethers are vinyl methyl ether and vinyl isobutylether. Preference is given to vinyl ethers of alcohols containing 1 to 4carbon atoms.

As hydrocarbons having 2 to 8 carbon atoms and two olefinic double bondsmention may be made of butadiene, isoprene, and chloroprene.

The addition polymer may further include monomers having carboxylic,sulfonic or phosphonic acid groups. Carboxylic acid groups arepreferred. Examples that may be mentioned include acrylic acid,methacrylic acid, itaconic acid, maleic acid, and fumaric acid.

Examples of further monomers include hydroxyl-containing monomers,(meth)acrylamide or phenyloxyethyl glycol mono(meth)acrylate, glycidylacrylate, glycidyl methacrylate, and amino(meth)acrylates such as2-aminoethyl(meth)acrylate.

It is essential that in total from 0.1 to 30% by weight of theethylenically unsaturated compounds of which the addition polymer iscomposed are the monomers A defined at the outset.

The monomers A) do not include monomers having carboxylic acid orcarboxylic anhydride groups.

Monomers A have a water solubility at 21° C. of more than 5 g of monomerper liter (1) of water.

The water solubility is preferably more than 10 g/l of water.

Customary acrylic monomers on the other hand, such as C₂-C₁₈alkyl(meth)acrylates, have a water solubility well below 5 g/l.

Preferred monomers A are selected from the acrylic monomers.Particularly suitable are hydroxyalkyl(meth)acrylates,methyl(meth)acrylate, (meth)acrylonitrile, and (meth)acrylamide.

Particular preference is given to hydroxyethyl(meth)acrylate,hydroxypropyl(meth)acrylate, methyl(meth)acrylate, (meth)acrylonitrile,and (meth)acrylamide.

Preferably from 0.5 to 20% by weight, with particular preference from 1to 12% by weight; of the monomers are monomers A.

The addition polymer preferably has a K value of from 30 to 80, withparticular preference from 40 to 60, measured in 1% solution (solvent:tetrahydrofuran, 21° C.).

The K value according to Fikentscher is a measure of the molecularweight and the viscosity of the polymer.

The glass transition temperature (T_(g)) of the addition polymer ispreferably from −60 to +10° C., with particular preference from −55 to0° C., with very particular preference from −55 to −20° C.

The glass transition temperature of the addition polymer can bedetermined by customary methods such as differential thermoanalysis ordifferential scanning calorimetry (see, e.g., ASTM 3418/82, midpointtemperature).

The UV-crosslinkable addition polymers can be prepared by copolymerizingthe monomer components using the customary polymerization initiatorsand, if desired, using regulators, polymerization being carried out atthe customary temperatures in bulk, in emulsion—for example, in water orliquid hydrocarbons—or in solution. The novel copolymers are preferablyprepared by polymerizing the monomers in solvents, especially insolvents with a boiling range of from 50 to 150° C., preferably from 60to 120° C., using the customary amounts of polymerization initiators,these amounts being generally from 0.01 to 10% by weight, in particularfrom 0.1 to 4% by weight, based on the overall weight of the monomers.Suitable solvents include in particular alcohols, such as methanol,ethanol, n- and isopropanol, n- and isobutanol, preferably isopropanoland/or isobutanol, and also hydrocarbons such as toluene and, inparticular, petroleum spirits with a boiling range of from 60 to 120° C.It is also possible to use ketones, such as acetone and methyl ethylketone, and esters, such as ethyl acetate, and also mixtures of suchsolvents, preference being given to mixtures containing isopropanoland/or isobutanol in amounts of from 5 to 95% by weight, in particularfrom 10 to 80% by weight, preferably from 25 to 60% by weight, based onthe solvent mixture used.

In the case of solution polymerization, appropriate polymerizationinitiators include, for example, azo compounds or ketone peroxides.

Following the polymerization in solution, the solvents can if desired beseparated off under reduced pressure, operating at elevated temperaturesin the range, for example, of from 100 to 150° C. In this case thepolymers can be used in the solvent-free state, i.e., as melts. In manycases it is also of advantage to prepare the novel UV-crosslinkablepolymers by polymerization in bulk, i.e., without the use of a solvent,in which case it is possible to operate batchwise or else continuously,in accordance, for example, with the information in U.S. Pat. No.4,042,768.

The addition polymer can be a melt, a solution in an organic solvent, oran aqueous dispersion and can be used in this form.

The polymers are preferably used in the form of a melt, i.e., inessentially solvent-free form (solvent content preferably less than 2%by weight based on the polymer).

The polymer can be applied to carriers, preferably from the melt, byusual techniques, such as brushing, rolling, flow coating, or knifecoating, for example. In the case of the solution or aqueous dispersion,the solvent or water is removed, generally by drying.

In order to increase the flowability of the polymer, the temperature ofthe polymer in the case of application as a melt can be from 10 to 150°C., preferably from 50 to 150° C., with particular preference from 100to 150° C.

Preferred film thicknesses are for example from 2 to 50 μm, withparticular preference from 5 to 40 μm, with very particular preferencefrom 10 to 30 μm.

Suitable carriers include, for example, labels made of paper or plastic,e.g., polyester, polyolefins or PVC, and also adhesive tapes or sheetsof the above plastics.

Subsequently, the polymers are crosslinked with high-energy radiation,preferably UV light.

For this purpose, the coated carriers are generally placed on a conveyorbelt and the conveyor belt is guided past a radiation source; forexample, a UV lamp.

The degree of crosslinking of the polymers depends on the intensity andduration of irradiation.

The radiation energy is preferably in total from 100 to 1500 mJ/cm² ofirradiated surface.

The resulting, polymer-coated carriers can be bonded to moistsubstrates, especially refrigerated substrates. These substrates may,for example, be frozen product packaged with paper or with polymerfilms.

Despite the moisture, and/or a film of water or layer of ice on thesubstrates, very good adhesion of the coated carriers, e.g., labels, isfound.

The addition polymer is therefore suitable as an adhesive, orpressure-sensitive adhesive, especially hot-melt pressure-sensitiveadhesive, for moist substrates, especially refrigerated substrates. Incomparison to hot-melt pressure-sensitive adhesives based onstyrene-butadiene(isoprene)-styrene block copolymers, the additionpolymers exhibit a much higher thermal stability and reducedstrikethrough, by which is meant the penetration of the adhesive coatingto the front face, the generally printed face of the carrier. In thecase of paper labels in particular, strikethrough results in an unwantedvisual deterioration.

EXAMPLES

I) Addition Polymers

P1: Acrylic polymer composed of 91% by weight ethylhexyl acrylate and 9%by weight hydroxyethyl acrylate

C1: Acrylic polymer composed of 100% by weight ethylhexyl acrylate

The polymers were prepared by conventional solution polymerization withsubsequent distillative removal of the solvent.

II) Preparation of the Coated Carriers and Testing

The polymers P1 and C1 were each coated from the melt onto label paperat a temperature of 120° C. The film thickness was 20 μm.

The coated label paper was irradiated with UV light and the polymer wascrosslinked.

The label paper was bonded to a polyethylene plate and the forcerequired for peeling, in N/25 mm, was determined using a tensile testingmachine.

The experiment was carried out at different temperatures of thepolyethylene surface:

+25° C. +5° C. −20° C. −10° C. * P1 4.2 5.8 3.7 3.6 C1 1.9 3.5 3.3 3.0 *The polyethylene surface was first wetted so that a coherent film ofwater formed.

We claim:
 1. A process for bonding a polymer coated carrier to arefrigerated substrate, said process comprising coating said carrierwith said polymer, and bonding said polymer coated carrier with saidrefrigerated substrate, wherein said polymer is a free radicallypolymerized, UV cross-linkable addition polymer, said polymer comprisesat least 50 wt. % of at least one C₂-C₁₈ alkyl(meth)acrylate and from0.1-30 wt. % of a polymerized monomer A, wherein said monomer A does notcontain carboxylic acid or carboxylic anhydride groups and has a watersolubility of more than 5 grams monomer per liter of water.
 2. Theprocess of claim 1, wherein said polymer comprises from 50-99.85 wt. %of at least one C₂-C₁₈ alkyl(meth)acrylate and from 0.05-10 wt. % ofsaid polymerized monomer A, which comprises at least one ethylenicallyunsaturated compound which has a photoinitiator group.
 3. The process asclaimed in claim 2 wherein the ethylenically unsaturated compound is anacetophenone or a benzophenone.
 4. The process of claim 1, wherein thepolymer has a K value of from 30-80 measured in 1% strength by weightsolution of the polymer in tetrahydrofuran at 21 C.
 5. The process ofclaim 1, wherein the polymer has a glass transition temperature of from−60 to +10 C.
 6. The process of claim 1, wherein the monomer A isselected from the group consisting of a hydroxyalkyl(meth)acrylate,methyl(meth)acrylate, (meth)acrylonitrile, (meth)acrylamide and mixturesthereof.
 7. The process as claimed in claim 1 wherein the polymer is amelt.
 8. The process of claim 1, wherein the carrier is first coatedwith the polymer to form a polymer coated carrier, then the polymer iscross linked by high-energy radiation, then the polymer coated carrieris bonded to a refrigerated substrate.
 9. The process of claim 8,wherein the carrier is a label, adhesive tape or sheet.
 10. The processof claim 18 wherein the high-energy radiation is UV light.
 11. A methodof applying a carrier to a refrigerated substrate, said methodcomprising applying a free radically polymerized, UV cross-linkablepolymer to said carrier, wherein said polymer is in a melted form, asolution or an aqueous dispersion, removing a solvent or water, ifpresent, then cross linking said polymer by high energy radiation, thenbonding the carrier, coated with a polymer to a refrigerated substrate,said polymer is a free radically polymerized, UV cross-linkable additionpolymer, said polymer comprises at least 50 wt. % of at least one C₂-C₁₈alkyl(meth)acrylate and from 0.1-30 wt. % of a polymerized monomer A,and wherein said monomer A does not contain carboxylic acid orcarboxylic anhydride groups and has a water solubility of more than 5grams monomer per liter of water.
 12. The method of claim 11 wherein thecarrier is a label, adhesive tape or sheet.
 13. A process for producinga refrigerated substrate, comprising: coating a carrier with a polymer,and bonding said polymer coated carrier with said refrigeratedsubstrate, wherein said polymer is a free radically polymerized, UVcross-linkable addition polymer, said polymer comprises at least 50 wt.% of at least one C₂-C₁₈ alkyl(meth)acrylate and from 0.1-30 wt. % of apolymerized monomer A, wherein said monomer A does not containcarboxylic acid or carboxylic anhydride groups and has a watersolubility of more than 5 grams monomer per liter of water.
 14. Theprocess of claim 13, wherein said polymer comprises from 50-99.85 wt. %of at least one C₂-C₁₈ alkyl(meth)acrylate and from 0.05-10 wt. % ofsaid polymerized monomer A, which comprises at least one ethylenicallyunsaturated compound which has a photoinitiator group.
 15. The processas claimed in claim 14, wherein the ethylenically unsaturated compoundis an acetophenone or a benzophenone.
 16. The process of claim 13,wherein the polymer has a K value of from 30-80 measured in 1% strengthby weight solution of the polymer in tetrahydrofuran at 21 C.
 17. Theprocess of claim 13, wherein the polymer has a glass transitiontemperature of from −60 to +10 C.
 18. The process of claim 13, whereinthe monomer A is selected from the group consisting of ahydroxyalkyl(meth)acrylate, methyl(meth)acrylate, (meth)acrylonitrile,(meth)acrylamide and mixtures thereof.
 19. The process as claimed inclaim 13, wherein the polymer is a melt.
 20. The process of claim 13,wherein the carrier is first coated with the polymer to form a polymercoated carrier, then the polymer is cross linked by high-energyradiation, then the polymer coated carrier is bonded to a refrigeratedsubstrate.
 21. The process of claim 20, wherein the carrier is a label,adhesive tape or sheet.
 22. The process of claim 20, wherein thehigh-energy radiation is UV light.